| 1. A fluoride-containing coating containing an element selected from the group consisting of Sm, Eu, Gd, Tb, Dy, Ho, Er, Y,
Tm, Yb and Lu and comprising a fluoride phase of the element which contains at least 50% of a crystalline phase of the orthorhombic
system belonging to space group Pnma as a primary crystalline phase on the basis of the entire crystalline phase,
the fluoride-containing coating being formed by a thermal spraying process comprising supplying a feed material in form of
powder to gas or plasma gas stream, melting or softening the material and depositing droplets thereof on a substrate, and
a heat treatment by holding the coating at a temperature in the range of 200 to 500° C. for at least 1 minute during or after
the thermal spraying.
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